Gene structure and transcription

Abstract

By following sequential cell-to-cell infection from the meristematic point back toward older cells, the chronological appearance of viral protein in barley stripe mosaic virus (BSMV)-infected wheat root and shoot tips was examined by immunoelectronmicroscopy. In infected root tips, progression of infection was followed in a series of the first 20 infected cells and divided into four cytological stages. In stage I, peripheral vesicles in proplastids acted as infection initiation markers. No BSM virions were seen, but viral protein could be first detected on vesiculated proplastid membranes by gold—IgG complexes at late stage I. Sequentially, BSM virions appeared mostly at right angles to the proplastid membrane at stage II and to the endoplasmic reticulum at stage III. In stage IV, but not before or after, viral protein appeared in abundance in nuclei while few or no virions were present. Viral protein occurred only in the euchromatin region and within fibrillar centers in nucleoli, never in the heterochromatin region. By gold—nuclease staining, euchromatin was the main location of nuclear DNA. No excess viral protein could be detected in cell extracts of systemically infected tissue. The chronological appearance of viral protein in infected shoot tips followed four infection stages in root tips. The presence of peripheral vesicles in proplastids could be used as infection initiation markers in root tips but not in shoot tips of infected plants; vesiculation was a normal occurrence in young proplastids of healthy and infected shoot tissue, did not occur in mature chloroplasts of healthy shoots, and was rarely seen in fully developed chloroplasts in infected shoot tissue.